[LoopVectorize] relax FMF constraint for FP induction

This makes the induction part of the loop vectorizer match the reduction part.
We do not need all of the fast-math-flags. For example, there are some that
clearly are not in play like arcp or afn.

If we want to make FMF constraints consistent across the IR optimizer, we
might want to add nsz too, but that's up for debate (users can't expect
associative FP math and preservation of sign-of-zero at the same time?).

The calling code was fixed to avoid miscompiles with:
1bee5497

Differential Revision: https://reviews.llvm.org/D98708